9.10 Disaster Recovery

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To ensure continued traffic flow in the event of an access link failure, some CSUDSUs provide failover services that automatically switch to an ISDN BRI or other standby link. This is useful in that the router or other CPE need not process and distribute route topology updates, which is typically a slow process. However, the disadvantage to handling failovers in such a transparent manner is that the router is not able to adjust route costs to reflect the link’s changed status.

Disaster recovery over frame relay links presents a set of problems that are addressed by CSU-DSUs specifically designed for frame relay. These links will not pass data if frame relay signaling is not present, so just having a dial-backup mechanism in place will not suffice—there must be some device that continues to behave like a frame relay switch, or the router will not send any data. On these occasions, the frame relay CSU-DSU will spoof the link management interface (LMI) to the router, accepting frames and transparently passing data around the failure point.

Some CSU-DSUs include line learning features, enabling them to automatically detect the characteristics of the underlying circuit such as line coding, framing, and the DS0 channels used. While this greatly simplifies installation, there is little deviation in most circuit configurations, especially those in the United States, where T1 lines typically use B8ZS line coding and ESF framing. Since manufacturers use these default settings, line learning may not be an essential feature.

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